Capillary Electrophoresis (CE) is a separation technique where a high voltage is applied to the sample inlet end of a glass capillary column and a lower voltage, or voltage of opposite polarity, is applied to the outlet end of the capillary. Analytes elute from the column at a rate that is determined by a combination of electroosmotic flow and the electrophoretic mobility of the analytes. Since the electroosmotic flow velocity can exceed the electrophoretic drift velocity of ions, it is possible to analyse both positive and negative ions in the same chromatographic separation. In such circumstances, the elution order can be generalized as multiply charged positive ions emerging first, followed by singly charged positive ions, followed by neutral analytes, followed by singly charged negative ions and finally followed by multiply charged negative ions. Commonly used CE detectors such as UV and fluorescence devices can analyse both ion polarities in a single chromatographic run.
However, when interfacing CE to mass spectrometry via an Electrospray ionization source, the column outlet is located at the Electrospray probe tip which in turn, is biased to typically 3 kV via a separate high voltage supply and a potential divider circuit. The analysis of positive and negative ions requires ESI tip voltages of +3 kV and −3 kV respectively. This precludes the use of fast positive/negative switching in a single chromatographic run since this would affect the total CE voltage and hence electroosmotic and electrophoretic flows.
Another disadvantage of the conventional arrangement is that biasing the ESI tip requires the additional cost of an ESI power supply circuit. Furthermore, such an arrangement imposes limitations on buffer concentration and ESI voltage stability.
It is therefore desired to provide an improved mass spectrometer.